Add hide hypervisor id on guest host

Image hide_hypervisor_id property helps libvirt set in xml instance file
property, which will hide on guest host KVM hypervisor signature
("KVMKVMKVM\0\0\0").
According to the commit message in QEMU repository [1]:

"The latest Nvidia driver (337.88) specifically checks
for KVM as the hypervisor and reports Code 43 for the
driver  in a Windows guest when found.  Removing or
changing the KVM signature is sufficient for the driver
to load and work."

[1]: http://git.qemu.org/?p=qemu.git;a=commitdiff;h=f522d2a

DocImpact

Implements: blueprint add-kvm-hidden-feature
Change-Id: I825c5c2a8ba3b9f5eccd9800870b6307d201614c

etc/metadefs/compute-libvirt-image.json

@@ -89,6 +89,12 @@
             "description": "Input devices allow interaction with a graphical framebuffer. For example to provide a graphic tablet for absolute cursor movement. Currently only supported by the KVM/QEMU hypervisor configuration and VNC or SPICE consoles must be enabled.",
             "type": "string",
             "enum": ["usbtablet"]
+        },
+        "img_hide_hypervisor_id": {
+            "title": "Hide hypervisor id",
+            "description": "Enables hiding the host hypervisor signature in the guest OS.",
+            "type": "string",
+            "enum": ["yes", "no"]
         }
     },
     "objects": []